Switching elements such as insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETs) made of silicon (Si) have been customarily included in power electronics equipment. In recent years, switching elements made of silicon carbide (SiC) having physical properties superior to those of Si have been actively developed.
The withstand voltage of MOSFETs depends on the thickness of the drift layer. SiC has a breakdown electric field higher than that of Si, and thus, the drift layer of a MOSFET made of SiC having a withstand voltage equal to that of a MOSFET made of Si can be thinner than the drift layer of the MOSFET made of Si. The ON resistance of the MOSFET is composed of, for example, channel resistance, JFET resistance, drift resistance, and substrate resistance. The SiC-MOSFET including a thinner drift layer as described above has a reduced drift resistance and thus achieves lower loss.
Unfortunately, currently used SiC-MOSFETs have an ON resistance that is still higher than the theoretical limit value of SiC. This is attributed to a higher channel resistance. Conventional SiC-MOSFETs are known to reduce the channel resistance by eliminating or reducing variations in channel length and thus achieving a miniscule channel length (see, for example, Non-Patent Document 1).